This invention relates to a tuning pulse generator for use in an electronic preset receiver, and more particularly to a tuning pulse generator of the type wherein tuning pulses are mechanically generated in accordance with the manual operation of a tuning knob.
Due to rapid and remarkable progress in the field of electronic techniques, various improvements for receivers have been proposed. Recently, one of such improvements is an electronic preset receiver in which a pulse generator is provided for generating up pulses and down pulses in accordance with the manual operation of the tuning knob. The output of the pulse generator is counted by an up-down counter, the output of which is digital-to-analog converted by a digital-to-analog converter to supply an analog signal to a varactor diode in the tuning section.
A conventional pulse generator is constructed as shown in FIG. 1 where the tuning knob 1 is directly connected to a disc 3 via a shaft so as to be interlocking therewith. The disc 3 has a plurality of slits 2 concentrically formed in its surface. A pair of light emitting elements 4a and 4b, e.g., light emitting diodes, are disposed at one side of the disc 3 spaced at some distance therefrom. Similarly, a pair of light receiving elements 5a and 5b, e.g., photo-transistors, are disposed at the opposite side thereof. The light emitted from the elements 4a and 4b is passed through the slits 2 and sensed by the elements 5a and 5b, respectively. The optical couplings between the elements 4a and 5a and between the elements 4b and 5b are both intermittent as the disc 3 turns. The thus constructed device generates up-down pulses and clock pulses by turning the tuning knob 1. The direction the disc 3 is turned determines whether up or down pulses are generated. In case that the up pulses are generated when the disc 3 is turned counterclockwise, the elements 5a and 5b are so arranged that the phase of the output of the element 5a is delayed by 90.degree. with respect to that of the element 5b. In this case, the outputs of the elements 5a and 5b provide up-down pulses and clock pulses, respectively. Owing to the turning direction of the disc 3, two kinds of output waveforms of the elements 5a and 5b are obtained as shown in FIGS. 6(a) and 6(b), in which FIG. 6(a) shows the case of up count and FIG. 6(b), down count. As can be appreciated from FIG. 6(a) the up-down pulse is at a high level in the case of up count whenever the clock pulse changes its level from low to high. Conversely, in the case of down count, the up-down pulse is at a low level whenever the clock pulse changes its level from low to high as shown in FIG. 6(b).
According to the tuning pulse generator thus constructed, however, various disadvantages are encountered. One of those is that the number of pulses generated with respect to one turn of the tuning knob is relatively small. Further, the counter sometimes misoperates or the operation of the counter is liable to be unstable due to the inner wall configuration of the slit as shown in FIG. 3. Specifically, since the area of the light spot on the disc 3 received from the light emitting element is much larger than the area of slit, a part of the irradiated light emitted therefrom impinges upon the inner wall of the slit and then the reflected light enters into the light receiving element. On the other hand, since the light receiving area in the light receiving element is larger than the area of the slit, the light receiving element receives the light even when the slit is slightly displaced. As a result, unwanted light is sensed by the elements 5a and/or 5b, the outputs of which are distorted in such a manner that the changing point of the output level from low to high and vice versa is smoothly rising or falling just like a sinusoidal waveform. Therefore, the prevention and the passage of light cannot be sharply carried out due to the inner wall configuration of the slit formed in parallel to the optical axis.